1. Field of the Invention
This invention relates to a coating composition which can cure by exposure to ultraviolet radiation, has capabilities of curing by exposure to moisture at room temperature, and has capabilities of curing by the application of heat. The ability to cure at room temperature or at elevated temperature provides the coating composition with a characteristic which allows it to be used for coating applications where the possibility of exposing all the area of a coated substrate with ultraviolet radiation are so unlikely, so impractical, that one may say it is impossible. This cure process which cures without the exposure to ultraviolet radiation can be termed "shadow cure" or "dark cure" meaning cure which occurs without being exposed to the ultraviolet radiation, i.e. cures in the shadow of the exposure or in the dark areas when the ultraviolet radiation is directed on the substrate.
2. Background Information
Compositions which cure by exposure to ultraviolet radiation are well known in the art, although the number of such materials found in commercial usage today are still relatively small. An even smaller number of such compositions are known to the art which both cure by exposure to ultraviolet radiation and also by another mechanism, such as by heat. Compositions which cure by two mechanisms are said to have dual cure.
Compositions which cure by exposure to ultraviolet radiation and by exposure to moisture, i.e. a dual cure, are also known in the art as shown by Lien et al in U.S. Pat. No. 4,528,081, issued Jul. 9, 1985. Lien et al describe a dual curing silicone composition which contains an acrylic functional dialkoxy or diaryloxy silyl group terminated polyorganosiloxane, an effective amount of a photosensitizer, and a silicone moisture curing catalyst. These compositions of Lien et al can also contain trimethylsilyl terminated silicone oil. Lien et al describe their materials as curing to soft gel like materials or soft rubbery materials. In their examples, the cured materials are defined as a soft rubbery material at 0% oil to a very soft sticky gel at 70% oil which showed a tendency to creep but would not flow.
Polyorganosiloxane compositions which have a dual cure mechanism, namely cure by exposure to ultraviolet radiation and by exposure to moisture at room temperature, are claimed in a copending application Ser. No. 07/200,827, filed Jun. 1, 1988, for Beth I. Gutek, entitled "COMPOSITIONS HAVING UV CURE WITH MOISTURE SHADOW CURE" and assigned to the same assignee as this application, which is hereby incorporated by reference to show the compositions which can be modified to make the present invention. Gutek in that application describes a composition comprising an acrylamide polyorganosiloxane, a polydiorganosiloxane having endgroups with two alkoxy groups to a silicon atom and a methacrylate group bonded to a silicon atom through an Si--C bond, a photoinitiator, and organic titanate. These compositions are best cured in an inert atmosphere during exposure to ultraviolet radiation.
Gutek also describes compositions which have a dual cure, namely cure by ultraviolet radiation and at room temperature by exposure to moisture, in copending application Ser. No. 07/118,086, filed Nov. 6, 1987, entitled "UV CURABLE CONFORMAL COATING WITH MOISTURE SHADOW CURE" and assigned to the same assignee as this application and is hereby incorporated by reference to show compositions which comprise acrylamide polyorganosiloxane, polydiorganosiloxane having alkoxy and methacrylate as described above, a reactive diluent selected from isobornyl acrylate, cyclohexyl acrylate, or 2-ethylhexyl acrylate, a combination photoinitiator which is a mixture of 2-hydroxy-2-methylphenylpropanone and 2-methyl-1-[4-(methylthio)phenyl]-2-morpholenopropanone, an amine selected from dimethylethanolamine or diisopropylethylamine, and an organic titanate. These compositions cure so fast in an inert atmosphere that it is necessary to cure them in the presence of air (oxygen). These compositions of Gutek can be modified to give the compositions of the present invention.
The compositions described in either of the Gutek copending application are very useful materials which can be used to coat printed wire boards as conformal coatings, but both have disadvantages, namely that the shelf life is not as long as needed to ship the materials to the coaters from the manufacturers plants and during the cure of these compositions the odor of the reactive diluent was very unpleasant and thus objectionable to the coaters. Investigations to resolve these problems lead to the unexpected discover of the compositions of the present invention.